I am not talking about specializing I am talking about mastering. Thats a very different thing.
You can specialize in playing a sweeping technique on your guitar that does not make you a good musician.
And again. No I simply don't believe you gain breadth by being exposed to many different things. It's the other way around. Learning a lot of different things superficially is not going to teach you the real lessons.
It's not about acquiring a specific set of skills it's about acquiring an ability to learn how to learn.
By exposing a wide range of activities to kids, they gain breadth. They can then choose what to explore in depth - that is their call.
"Overspecialize, and you breed in weakness. It's slow death."